This invention concerns a coating composition for the rear side of photographic support materials for light-sensitive layers and a process for producing same. The support material may be a plastic coated paper or a plastic foil. The rear side is the surface of the support material opposite the front side which will later carry the image.
Plastic foils or papers coated on their surfaces, i.e. the front side and rear side, with polyolefin layers are usually used as support materials for photosensitive layers, especially for color photography. These polyolefin coatings are preferably applied by melt extrusion methods using polyolefin. Such a photographic support materials are described, for example, in U.S. Pat. No. 3,411,908.
The polyolefins may be polyethylenes, such as LDPE, LLDPE, HDPE or polypropylene or mixtures of these components. They have many advantages as layer forming substances, but they also have properties that are a disadvantage within the context of producing or further handling of such papers. Apart from the adhesion problems between the polyolefin surface and the light-sensitive emulsion which can be eliminated by additional measures and means, polyolefin layers must fulfill certain properties in the course of production and processing of such photographic papers or, in some cases, certain properties of the polyolefins must be suppressed or overcome.
Thus, for example, it is necessary to be able to label the photographic materials that are to be developed by writing or printing on the rear side of the support material so the materials can be identified as belonging to certain customers and clients. A hydrophobic polyolefin coating that seals the paper core, however, permits writing or labeling only to a limited extent. Special measures and means are required in order to assure general printability or writability in machine processing of the photographic material.
It is also necessary for the photographic material coated with light sensitive emulsions not to attract dirt particles to its surface ("tar stain") in the various treatment baths. These stains develop over a period of time due to oxidation and condensation processes in old photographic treatment baths.
In addition to such partial reception of dirt particles at the surface (tar stain) there can also be absorption of developer solution on the full area of the support material to such an extent that it is no longer completely reversible during the further passage through the treatment baths. This absorbed developer or the absorbed developer components oxidize in air and lead to more or less intense yellow discoloration of the full surface area.
In addition, it is necessary to prevent the developer solutions or baths from becoming contaminated by the agents that are responsible for printability, writability and similar desirable properties. This means that the layer on the rear side (backing layer) must be abrasion resistant and bath resistant.
Another requirement for such photographic support materials is good adhesion for adhesive tape which is used to secure the rolls of photographic paper strips to each other. The tape connections must not become detached when passed through the aqueous bath liquids in the developing process.
Finally, electrostatic charge buildup by the support materials should also be prevented in order to prevent flash exposure as the plastic coated photographic support material or plastic foil is passed through the emulsion coating machine or through the developing machines. Such flash exposure would lead to the non-usability of the light-sensitive emulsions or could destroy the latent image that is developing.
A new requirement that has been added is printability with thermal printers. Thermal printers transfer printing inks within a short period of time at high temperatures.
In summary, a backing layer should impart the properties of writability, printability, adhesive tape adhesion, abrasion resistance, antistatic properties and thermal printability to the support materials for light-sensitive layers, but should not contaminate the photographic treatment baths, should not allow reception of dirt due to tar-like oxidation products from the photographic developers, and should not allow discoloration of the surface due to oxidized developer components.
It is known that the requirements of a rear backing layer described here necessitates different measures, and in the past it has been difficult to achieve these requirements on the whole because the measures and means presented to solve them were often contradictory.
For example, it is known from European laid open Publication No. 0 160 912 that the rear side of a polyethylene coated photographic material can be provided with an antistatic layer consisting of a sodium magnesium silicate, a sodium polystyrene sulfonate and certain succinic acid semiesters. This layer should prevent a buildup of electrostatic charge and should also protect the material from dirt. However, this layer does not permit good adhesion of adhesive tape, has a low bath resistance and unsatisfactory thermal printability.
DE-OS 3 700 183 discloses a photographic support material with a backing layer that is a useful compromise between different good properties. It has good antistatic properties, good printability, moderate to good abrasion resistance and bath resistance, low to moderate dirt reception and good adhesive tape adhesion. However, the dirt reception (tar stain) in various other oxidized photographic developers is too great, and the thermal printability is unsatisfactory.
EP-OS 312 638 describes a photographic support material for light-sensitive layers with a similar backing layer as that in DE-OS 3 700 183. It yields good results in antistatic properties, adhesive tape adhesion, printability, abrasion resistance and bath resistance, and also has minimum tar stain and discoloration of the surface due to oxidized developer components. In addition, it can be written on well with a pencil. However, this backing layer is poor with regard to thermal printability.
All the solutions described here, however, have proven to be inadequate in recent times because due to a constant increase in the use of developer capacities, the pot life (changing cycle) of photographic baths has become longer, and tar-like and strongly colored oxidation products are formed to an increasing extent in the photographic developing baths. Consequently, higher and higher demands are made of the photographic support materials, especially with regard to tar stain and surface discoloration.
Another requirement regarding the rear side of photographic support materials is that it must be printable with thermal printers where a printing ink is transferred within a short period of time at a high temperature.
Therefore, the object of this invention is to make available coating compositions for backing layers of support materials for light-sensitive layers that also have significantly reduced soiling due to tar-like oxidation products from the photographic developer bath (tar stain) after passing through extremely aged photographic treatment baths, definitely lower discoloration due to oxidizing developer components absorbed at the surface than in the current state of the art, and they must be printable with thermal printers. A basic condition here is that the layer must not be at all inferior to the state of the art with regard to the other properties described above.
This object is solved by an aqueous coating composition containing the following components:
a colloidal aluminum modified silica,
an alkali salt of an organic polyacid,
a polyfunctional aziridine, and
a plastic dispersion that has
a residual monomer content of &lt;200 ppm, PA1 &gt;50 mol% rigid monomers in the polymer, PA1 free carboxyl groups in the polymer, and PA1 an interfacial tension of &gt;50 dyn/cm as the dried film.
However, this basic formulation can be supplemented by additional additives in order to reinforce existing properties or create other properties. Such additives may include a silica with a particle size of &gt;2 .mu.m, optical brighteners, nuancing dyes, delustering agents of an organic or inorganic type, white pigments, wetting agents, etc. For example, adding silica with a particle size of &gt;2 .mu.m makes the layer more suitable for accepting pencil writing.
The individual components in the aqueous coating composition are present in the following amounts:
______________________________________ plastic dispersion 3.0-7.0 wt % (as a 50 wt % aqueous dispersion) aluminum modified colloidal silica 2.5-9.0 wt % (as a 30 wt % aqueous dispersion) alkali salt of an organic polyacid 0.5-4.0 wt % (as a 30 wt % aqueous dispersion) polyfunctional aziridine 0.2-1.5 wt % (as a 50 wt % alcoholic solution) with the remainder being water. ______________________________________
Then the dried layer contains approximately the following amounts:
______________________________________ polymer 30-70 wt % aluminum modified silica 16-64 wt % alkali salt of an organic polyacid 3-25 wt % polyfunctional aziridine 2-16 wt % ______________________________________
The polymer of the plastic dispersion is thus composed of one or more of the monomers styrene, butadiene, acrylic ester, methacrylate ester, vinyl acetate, vinyl chloride, maleic ester, olefin or acrylonitrile combined with monomers with free carboxyl groups. These monomers with free carboxyl groups may include, for example, maleic acid, acrylic acid and methacrylic acid, crotonic acid, itaconic acid, vinyl acetic acid, etc.
A copolymer of the aforementioned monomers is preferred. There should be &gt;50 mol% rigid components such as styrene, methacrylate ester, acrylate ester, vinyl chloride or vinyl acetate in the copolymer. These are components or monomers which in the form of homopolymers have a ball indentation hardness of &gt;1000 kg/cm.sup.2. The copolymer must have a low free carboxyl group content for the crosslinking reaction with polyfunctional aziridines. This amount should be preferably 1-5 mol%. The residual monomer content should be &lt;200 ppm.
It has surprisingly been found that coating compositions with selected plastic dispersions having an interfacial tension of &gt;50 dyn/cm as the dried film meet all the required properties, whereas coating compositions with plastic dispersions that have an interfacial tension of &lt;50 dyn/cm as the dried film yield unsatisfactory results.
The aluminum modified silica in the coating composition has, as a colloidal solution, a particle size of 7-16 nm and the modification depends preferably on exchanging a few silicon atoms for aluminum atoms.
The alkali salt of an organic polyacid may be a lithium salt, a sodium salt or a potassium salt of polyacrylic acid or polymethacrylic acid, maleic acid, itaconic acid, crotonic acid, polysulfonic acid or copolymers of these compounds as well as cellulose derivatives. The alkali salts of polystyrene sulfonic acid or naphthalene sulfonic acid or an alkali cellulose sulfate are preferred.
Of the polyfunctional aziridines, trifunctional aziridines of the following general formula are especially preferred: ##STR1## where R.sub.1 is --CH.sub.3 or --OH and R.sub.2 is --CH.sub.3 or --OH.
All conventional applicator systems are suitable for applying the coating compositions. The surface of the photographic support material to be coated in this way is preferably pretreated by corona discharge in order to achieve better adhesion of the applied layer.
The following examples are presented to illustrate this invention in greater detail but do not restrict in any way.